Session L52: Magnetic Topological Materials 1: Mn-Bi-Te

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It was recently demonstrated that the layered van der Waals bonded material MnBi₂Te₄ is an intrinsic antiferromagnetic topological insulator. The opening of an electronic gap in the surface state, originating in the presence of exchange interaction, was experimentally verified by ARPES. However, the presence and magnitude of this gap are still under debate. To develop a comprehensive understanding of this class of materials and ultimately achieve control over their topological phases, more experimental characterization of their spatial heterogeneity is needed.
In this talk we discuss low-temperature scanning tunneling microscopy and spectroscopy measurements of MnBi₂Te₄. We first use topographic maps to identify the surface profile, including steps which reflect the septuple-layer structure. Using scanning tunneling spectroscopy, we probe the local density of states and identify a bandgap with the same magnitude as some recent ARPES reports. We observe spatial inhomogeneity in the DOS which could be responsible for the reported differences in the size of this surface state gap. Using spectroscopic maps we characterize the electronic states associated with the presence of edges in the surface.   

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Magnetic topological insulators (MTIs) have become an area of interest due to the opening of an exchange gap which could allow for the study of topological behavior at elevated temperatures. Additionally, they have been predicted to host several intriguing phenomena such as the quantum anomalous hall effect, magneto-electric effects, and chiral topological edge states. MnBi₂Se₄ (MBS) is a predicted MTI formed of van der Waals separated septuple layers (SL) with a layered anti-ferromagnetic structure. We used scanning tunneling microscopy to study a 20 SL MBS film grown using molecular beam epitaxy and observed two distinct surface terminations, Se and Bi, which we characterized with scanning tunneling spectroscopy (STS) and differential conductance mapping. STS revealed a gap-like local density of states (LDOS) for the Se-termination, and a more metallic LDOS on the Bi-termination. We further investigated the Se-termination by imaging the atomic structure as well as Mn_Bi antisites defects that manifest as threefold symmetric objects in the Se layer. Finally, spatially resolved STS was used to probe the evolution of electronic structure near step edges, and we find in-gap conductance localized to 1SL step edges that could reflect the predicted topological edge states.   

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Interfacing ferromagnetic insulators with 3D topological insulators offers the potential to engineer heterostructures that can host the Quantum Anomalous Hall (QAH) insulator and Topological Magnetoelectric effect phases. These phases are realised as a result of magnetisation that breaks time reversal symmetry and opens up an exchange gap. Single septuple layer (SL) of MnBi₂Te₄ (MBT) has recently been reported to be a 2D ferromagnetic insulator possessing a bandgap >700 meV with long-range ferromagnetic order.¹ Its chemical and structural similarity to the 3D topological insulator Bi₂Te₃, has led to predictions that MBT/BT/MBT heterostructures may yield robust QAH insulator phases with bandgaps >50 meV.² Here we demonstrate the growth of 1SL MnBi₂Te₄ and 4QL Bi₂Te₃ heterostructures via MBE, and probe the electronic structure using angle resolved photoelectron spectroscopy. We observe massive Dirac Fermions with band gaps >50meV and strong hexagonal warping of the Dirac cone, in excellent agreement with theory. The engineering of massive Dirac Fermions and the QAH insulator phase in MBT/BT/MBT heterostructures has potential applications in spintronics and quantum computing.
¹ Trang et al., arXiv:2009.06175
² Otrokov et al., 2D Mater. 4, 025082 (2017)   

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The (MnBi₂Te₄)(Bi₂Te₃)_m (m=0, 1, 2, &3) system provides a rich platform for the study of various exotic quantum states, such as axion insulator (AI) and quantum anomalous Hall insulator (QAHI). With m increasing, the strength of interlayer magnetic exchange interaction gradually weakens. Previous studies have shown the magnetic state of this system changes from antiferromagnetic (AFM) orders for m=0, 1 and 2 to a ferromagnetic order (FM) for m=3. The m=2 member, MnBi₆Te₁₀ exhibits enhanced competition between AFM and FM coupling, and a FM order has been reported to be accessible via 30% Sb substitution for Bi [1]. In this talk, we report the successful growth of FM Mn(Bi_1-xSb_x)₆Te₁₀ (x = 0, 0.08, 0.12, 0.3) crystals. Some samples exhibit a large remnant magnetization (2.7 μ_B/Mn). Our experiments also reveal a large anomalous Hall effect for the x=0.3 sample; its anomalous Hall resistivity is ~ 10 μΩ cm, about 10 times larger than that previously reported value for the AFM MnBi₆Te₁₀. These FM compounds could be promising candidates for realizing the QAHI and AI.

Reference:
[1] J. Wu, F. Liu, C. Liu, Y. Wang, C. Li, Y. Lu, S. Matsuishi, and H. Hosono, Adv. Mater. 32, 2001815 (2020).   

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MnBi₂Te₄ is one of the most promising candidates of intrinsic magnetic topological insulators (MTIs) that displays nontrivial band topology with an intrinsic antiferromagnetic state. However, there are inconsistent observations on the existence of the exchange gap of MnBi₂Te₄, partially due to the highly electron-doped nature of the MnBi₂Te₄ crystals and local inhomogeneity from the native defects. Here, we tailor the material through Sb substitution to reveal the gapped surface states in MnBi_2−xSb_xTe₄. By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism detected by in situ magnetostriction measurement. The realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.   

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MnBi_2+2nTe_4+3n are promising topological insulators (TI) where the natural intergrowth of magnetic layers and TI layers provides a unique platform for the study of interplay between magnetism and topological electronic states. Here we present our inelastic neutron scattering (INS) study on single crystals MnBi₂Te₄ and MnBi₄Te₇. We find that antiferromagnetic (AF) interlayer magnetic interactions in MnBi₂Te₄ which are vanishingly small in MnBi₄Te₇, need to be anisotropic for a consistent description of both INS and magnetization measurements. The modeling of two-dimensional intralayer ferromagnetic (FM) spin waves requires introduction of long-range and competing FM and AF magnetic interactions up to the seventh nearest neighbor. First principles calculations of insulating MnBi₂Te₄ support this model. The abnormal spin wave peak widths observed with INS could result from Mn vacancies/ Mn-Bi anti-site exchange as suggested by our spin dynamics simulation.   

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We applied Raman spectroscopy to investigate lattice vibrational modes in the magnetic topological insulator MnBi₂Te₄ with thicknesses ranging from 1 to 5 septuple layers. We identified the symmetry of phonon modes based on polarization dependent measurements. Also, we observed thickness-dependent phonon modes at frequencies below 12cm^-1. Temperature and magnetic field dependent measurements will be also discussed.   

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Intrinsic magnetic topological insulators (TIs) MnBi_2nTe_3n+1 provides a promising material platform to realize exotic topological quantum states such as high temperature quantum anomalous Hall insulator and axion insulator. Besides non-trivial band topology, magnetism is another necessary ingredient to realize these quantum states[1-5]. In this talk, we will show a ferromagnetic (FM) phase with T_C~14 K can be realized with a small concentration substitution of Sb for Bi in Mn(Bi,Sb)₄Te₇. We have also observed strong competition between interlayer FM and antiferromagnetic (AFM) coupling. In pristine MnBi₄Te₇, we observed an AFM transition at 13.1 K, followed by a FM transition at 8.9 K, consistent with previous reports[6]. However, in a 60%Sb doped sample, we find a FM phase switches to an AFM phase at ~4.4K. These complex magnetic transitions offer rich opportunities to observe how the topological phases evolve with magnetism. We will also discuss the origin of the AFM and FM competition in this material system.
1.Zhang et al.,PRL,122,206401(2019)
2.Li et al.,Sci. Adv.5,eaaw5685(2019)
3.Otrokov et al.,Nature 576,416(2019)
4.Hu et al.,Nat. Commun.11,97(2020)
5.Yan et al.,PRM 4,054202(2020)
6.Vidal et al.,PRX 9,041065(2019)   

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MnBi₂Te₄ is an emergent material platform for exploring topological phenomena owing to its intrinsic magnetic order combined with nontrivial topology. Bulk crystal has an A-type antiferromagnetic (AFM) order below its Neel temperature, while the layer dependent magnetism of thin flakes remains largely unexplored. It is unclear how the bulk band structure changes when the magnetic states are tuned by an external field, which is crucial for understanding the topological phase transition in this system. In this work, we probe the intrinsic magnetism of atomically thin MnBi₂Te₄ samples by reflective magnetic circular dichroism (RMCD), and correlate their magnetic order with transport measurements. We show that in thin flake the remnant magnetization and critical spin-flop field exhibit even-odd layer number effects. For both even and odd layers, applying a magnetic field drives the layered AFM state into a canted AFM state, and finally a ferromagnetic (FM) state. The tuning of magnetic states corresponds to concurrent topological phase transitions with a Chern number from C=0 to C=-1.   

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The Weyl semimetal in a constant magnetic field is studied. The ferromagnetic interaction is projected to the lowest (n=0) Landau level. The higher Landau levels are considered at the one-loop approximation and renormalize the n=0 ferromagnetic interaction. This theory, for a Weyl semimetal with two nodes in the direction of the magnetic field, gives rise to a one-dimensional sliding charge-density-wave (CDW) when the chemical potential is above the highest occupied Landau level (in the insulating phase), and is absent in the metallicphase. Our results are in agreement with the experimental findings of J. Gooth et al. [Nature 575, 315 (2019)].   

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The recently discovered MnBi₂Te₄ combines intrinsic magnetism and nontrivial topology in one material, providing an ideal platform for exploring novel topological phases. In this talk, we report transport studies of exfoliated MnBi₂Te₄ flakes with varied layer structure and magnetic fields. In the 6-SL (septuple layer) sample tuned into the bulk-insulating regime, a robust zero Hall plateau and insulating longitudinal resistance exist in a wide range of magnetic fields and gate voltages. These are transport characteristics of an axion insulator when the top and bottom surfaces of MnBi₂Te₄ have opposite magnetization. An external magnetic field polarizes the magnetization of all the layers, and drives a transition from the axion insulator phase to a Chern insulator with zero longitudinal resistance and quantized Hall resistance. In a 7-SL sample subjected to pulsed magnetic field up to 60 T, we observe systematic and yet uniquely complex evolution of quantized Hall plateaus with Chern numbers from C = -3 to +1. More surprisingly, a novel phase characterized by an extremely broad zero Hall plateau emerges as the most robust ground state in the high field limit. Non-local transport measurements and theoretical calculations reveal that this C = 0 phase arises from the coexistence of a ferromagnetic-order-induced Chern band with C = -1 and a Zeeman-effect-induced Chern band with C = +1. This helical Chern insulator phase with broken time-reversal-symmetry represents a new type of quantum Hall effects originated from topologically nontrivial band structure.   

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Intrinsic magnetic topological insulators have been extensively studied due to the theoretical prediction that it will provide a platform for novel topological phases, such as the quantum anomalous Hall effect (QAHE), the axion insulators (AIs), and the type-II magnetic Weyl semimetals depending on thickness. Recently, the QAHE and AI have been observed on MnBi₂Te₄ flakes with odd and even number of layers, respectively. However, further progress on this novel material is hindered by the difficulty in preparing its high-quality thin films with well-controlled compositions and thickness. Here we report on the magnetic properties of MnBi₂Te₄ grown by MBE with various thicknesses. We observed two main results: magnetic phase transitions and the linear dependence between the Hall conductance and the magnetization. Both transport and SQUID measurements clearly show the magnetic phase transitions between a FM to an AFM state and vice versa, which brings the possibility of hosting new topological phases on MBE grown samples. In addition, we verify the origin of the AHE of MnBi₂Te₄ is intrinsic, from its linear dependence of magnetization. Our results enable the realization of QAHE in the large area MnBi₂Te₄ films with thickness precisely controlled by MBE.